In healthy humans, sensory systems are intricately organized in the cortex where specific areas are dedicated to processing specific sensory input (known as the cortical sensory map). This phenomenon holds true in the sensory and motor domains (known as somatotopic cortical representation). For example, stimulation of the index finger excites a very distinct portion of the cortex while stimulation of the middle finger excites a different distinct part of the cortex. Many studies have shown that somatotopic cortical representation is plastic and can change in response to environmental exposure and learning (termed brain plasticity). For example, rats exposed to an impoverished environment showed a degradation of the somatosensory cortical map (Coq & Xerri, 1999) whereas rats exposed to an enriched tactile environment showed a refinement of sensory representation (Coq & Xerri, 1998). Thus, environmental factors can lead to positive or negative plastic changes in the fidelity of representations in cortical maps.
Similar to the effects seen in rats, monkeys who trained on a tactile discrimination task demonstrated refined cortical receptive fields for the portion of the finger that was trained (Recanzone, Merzenich, Jenkins, Grajski, & Dinse, 1992). Additionally, it has been shown that somatotopic changes likely contribute to improved performance in the task (Recanzone, Jenkins, Hradek, & Merzenich, 1992). These findings suggest that refined sensory representation of peripheral effectors (hands, fingers, etc.) may lead to improved motor performance. Conversely, some have hypothesized that certain motor impairments are a result of negative neural re-organization as a response to learned behaviors (Byl et al., 1997; Lenz & Byl, 1999; Lenz et al., 1999; Lim, Altenmuller, & Bradshaw, 2001). A profound example of impaired motor control is hand function (manual dexterity) of older adults. It is widely accepted that older adults demonstrate reduced manual dexterity. A reduction in manual dexterity cannot be entirely accounted for by cutaneous sensory impairment, mechanoreceptor impairments or peripheral effector ailments such as arthritis or diabetic neuropathy (Cole, Rotella, & Harper, 1999). Since impairments in hand function cannot be entirely explained by problems in the hand itself, changes in higher control centers within the nervous system must account for the remaining reduced dexterity seen with age.
Healthy adults are remarkably adept at using their hands to manipulate a diversity of objects, adjusting force output to various object properties with limited conscious awareness. The manner in which forces are adjusted is dependent on appropriate integration of the sensory and motor systems (Johansson, 1996; Johansson & Cole, 1992; Johansson & Westling, 1991). As an object is manipulated with the hand, force output is calibrated by information provided by mechanoreceptors in the joints, muscles and tendons as well tactile sensory feedback from the skin. The typical result of this feedback loop is skillful object manipulation. However, when sensory-motor integration goes awry, the result is impaired motor control. Various examples of this occur in normal aging. For example it has been shown that age related changes in subcortical (Emborg et al., 1998) and cortical (Ward & Frackowiak, 2003) areas correlate with age-related changes in motor function of the hand. Some of the changes in the nervous system that contributed to reduced manual dexterity are due to negative plasticity. With an appropriate sensory-motor training program, some or all of the manual dexterity that is lost to negative plasticity can be recovered.
A prerequisite of skillful object manipulation (manual dexterity) is adequate and economic modulation of prehensile force output. This is accomplished through an integration between somatosensory input and motor output where the forces applied to an object are calibrated based on information provided by mechanoreceptors of the hand and tactile sensors on the skin (R. S. Johansson, 1996; R. S. Johansson & Cole, 1992; R. S. Johansson, Riso, Hager, & Backstrom, 1992; R. S. Johansson & Westling, 1984, 1987, 1991; Sober & Sabes, 2003). Sensory-motor integration occurs at several levels in the nervous system. It has been demonstrated that the nervous system is plastic and can change in response to environmental exposure and learning (termed brain plasticity). When neural reorganization occurs in the sensory-motor pathways, the result is either improved motor control (Bourgeon, Xerri, & Coq, 2004; Tegenthoff et al., 2005) or aberrant motor control (Byl et al., 1997; Byl, Nagarajan, & McKenzie, 2003; Byl, Nagarajan, Merzenich, Roberts, & McKenzie, 2002). Brain plasticity resulting to improved performance can be referred to as positive plasticity, whereas brain plasticity resulting in impaired performance can be referred to as negative plasticity.
The hand is especially sensitive to changes in neural organization. For example, monkeys exposed to limited tactile stimulation resulted in cortical reorganization of both the stimulated and non-stimulated portions of the hand (Jenkins, Merzenich, Ochs, Allard, & Guic-Robles, 1990). There is evidence that a refined somatotopic map leads to improved performance in sensorimotor tasks (Recanzone, Jenkins, Hradek, & Merzenich, 1992) and that degraded somatotopic organization results in impaired performance (Cole, Rotella, & Harper, 1999; Lenz & Byl, 1999; Lenz et al., 1999).
Cortical organization is partially determined by a competition of stimuli, with the stimulus that contributes the most input winning cortical real estate. For example, if a person engages in a lifestyle where a majority of the neural input from his/her hand is a small focal area, then that small focal area will dominate the resources for neural representation and will received expanded representation. Some examples of this are found in Braille readers, string musicians, and pianists. These activities result in an expansion of the pad of the fingertip and a dedifferentiation of the digits.
Humans engage in a multitude of activities that may negatively effect cortical organization. People who receive limited tactile stimulation of their fingers, for example, the above mentioned string musicians or pianists, are often afflicted with an ailment known as Focal Hand Dystonia (FHD), which is a disorder of neuroplasticity that results in abnormal motor control, specifically involuntary spastic contractions of the hand and fingers. Said another way, negative plasticity is partially caused by stereotyped isolated stimulation of one portion of the hand (typically the pad of the finger tip). When this occurs, the representation of the portion of the hand that is highly stimulated dominates the representation of the entire hand. The result is heightened representation of the stimulated part and depressed representation of the remaining un-stimulated portions, which results in decreased sensory-guided fine motor control.
Such loss of sensory-guided fine motor control, due to degraded sensory maps, may substantially degrade the capabilities of people who might otherwise operate normally in their daily life, and thus may negatively affect their quality of life, e.g., professional functionality, recreation, etc.
Thus, improved systems and methods for improving sensory-guided fine motor control of the hand are desired.